The present invention relates to a process for treating residual silicon powder, and more particularly to a process for the passivation of residual silicon powders or fines, particularly those silicon containing powders resulting from the manufacture of halosilanes, organohalosilanes, organoalkoxysilanes, and the like. The present invention relates to the passivation, inactivation, or deactivation of the silicon fines from the fluidized bed manufacture of halo silanes, organohalosilanes, organoalkoxysilanes, and the like. The basic process for the manufacture of such silane compounds is well-known and is described in U.S. Pat. No. 2,380,995. The present invention is particularly directed to the fluidized bed process for the manufacture of halosilanes using a fluidized bed reactor similar to if not identical with the teachings as taught in U.S. Pat. No. 2,389,931. While the scope of the present invention is particularly directed to the fines generated from a fluidized bed reaction process to produce the halosilanes, organohalosilanes or organoalkoxysilanes other types of reactors used for the production of these compounds also generate fines similar in nature and composition to the fines passivated by the present invention. Some of these other reactor types involve, for example, stirred bed or fixed bed reactors in contrast to fluidized bed reactors.
Organotrichlorosilanes and diorganodichlorosilanes are two of the basic products of the above described direct process reaction. Such compounds are utilized in the production of organopolysiloxane resins as described in U.S. Pat. Nos. 2,258,218 through 2,258,222. Other products include organopolysiloxane fluids as described in U.S. Pat. Nos. 2,469,888 and 2,469,890 as well as the organopolysiloxane elastomers described in U.S. Pat. No. 2,448,756. The siloxane compounds produced by these reactions are generally used to produce linear polysiloxane fluids and polymers in the production of heat cured rubber elastomers and room temperature vulcanizable silicone rubber compositions of various types. Thus the silanes produced by the reaction process previously referenced become siloxanes and organopolysiloxane resins eventually becoming silicone rubbers and elastomers and the like.
The chemical grade silicon typical of reactor feed to the manufacturing processes referenced above has a percent silicon that is typically greater than 98% and contains to a small degree by Fe, Al, Ca, Ti and other elements making up the remainder of the composition. These trace elements are concentrated as the reaction consumes silicon to produce or manufacture the halosilanes, organohalosilanes or organoalkoxysilanes As the reaction proceeds in the fluidized bed manufacture of methylchlorosilane and other substituted silanes, small particles or fines containing silicon are ejected from the fluidized bed reactors during normal operation. Many of these particles are carried along with the crude or unpurified methylchlorosilane. The fine silicon containing particles that are captured may contain among the various constituents of the fines, reactive metals, chlorosilanes, and hydrocarbons associated with them. Prior to disposing of these fines, they must be reacted or otherwise passivated, inactivated, or deactivated before they may be safely handled and disposed. The materials must be rendered non-reactive with respect to contact with air, water or other reactive media that might be encountered by the fines.
Much of the art associated with the processing of silicon containing fines from the manufacture of organochlorosilanes has been associated with increasing the fraction of silicon reacted to form the products and increasing the efficiency of the silicon reaction in the fluidized bed reactor process. Thus there are several processes in the art for utilizing the residual silicon particles and recycling them to utilize the unreacted silicon contained therein within the process reactor train. The present process addresses the processing of these fines once they have left the silane manufacturing reactor train.
U.S. Pat. No. 4,307,242 teaches a process for recovering and recycling silicon containing fines in an organochlorosilane reactor system. The process of the '242 patent comprises a method for classifying direct process contact mass by particle size whereby the least selective, more poisoned, or impure silicon containing particles are separated from those silicon containing particles that are relatively active and selective, relatively unpoisoned, and relatively pure thereby improving the usefulness of the silicon. In the '242 patent fine effluent powder (residual contact mass or residual silicon) is directed to one or more mechanical cyclones for recovery. In the '242 patent, this fine effluent powder is generally the spent reaction mass from a reactor that produces organotrichlorosilane and diorganodichlorosilane products. According to the '242 patent, crude or impure organotrichlorosilane and diorganodichlorosilane products are recovered from the top of the cyclones and these products may contain small amounts of "very fine" entrained particles therein. The remainder of the reaction mass is treated pneumatically in the mechanical cyclones and is directed to a receiving hopper for alternate disposition. Unless the spent reaction mass is being recycled to the reactors, the '242 patent is silent on the question of alternative methods of disposing of spent reaction masses.
A method of treating spent metallic reaction masses produced in the direct process production of organohalosilanes is taught in U.S. Pat. No. 2,803,521 wherein spent, i.e. residual, silicon-containing reaction masses are dispersed in water or dilute hydrochloric acid and contacted with a chloride source at a temperature ranging from 20.degree. to 100.degree. C., treated with the chloride containing solution and the silicon particles allowed to settle out while the supernatant solution is treated to precipitate the dissolved metal salts and the precipitated salts are collected and re-used as fresh catalyst for the process.
Current methods involve mixing the fines with water and a binder with the object of rendering the material safe for handling. When contacted with water the fines generate hydrogen gas. Current experience is that the process is erratic, unreliable, and potentially dangerous due to the evolution of hydrogen gas. A particular problem is that the process produces lumps of unreacted fines sealed by an outer layer of reacted fines. Later rupture of these lumps exposes untreated material that may lead to a violent reaction with water. Similar problems are encountered in the briquetting operation. Thus, there are significant contacting problems associated with a simple water treatment of the fines.